The present disclosure relates generally to an apparatus and method for producing a calibrated Raman spectrum, and particularly to an apparatus and method for producing a calibrated Raman spectrum using a test standard.
Spectroscopy refers to the study of energy or intensity as a function of wavelength in a beam of light or radiation. Raman spectroscopy refers to the study of the wavelength and intensity of inelastically scattered light from molecules, and an analytical technique that may be used for the analysis of covalently bound chemical substances found on surfaces or bulk materials. When a material is exposed to monochromatic radiation, a phenomenon known as Raman scattering results, which produces Raman spectra having frequencies that are characteristic of the exposed molecule and the various groups and bonds in the molecule. Raman scattered light is frequency-shifted with respect to the incident light excitation frequency by the energies of the molecular vibrations, and since the magnitude of the shift is independent of the excitation frequency, the resulting “Raman shift” is illustrative of an intrinsic property of the sample under test.
The Raman scattering effect is typically very weak and Raman spectrometers must be capable of separating the weak inelastically scattered light from the intense elastically scattered incident laser light. As a result, apparatus for producing Raman spectrum are sensitive to variations within the test apparatus itself, the test environment and the test sample.
In an effort to resolve some of the difficulties associated with Raman spectroscopy, several areas have been investigated for reducing system variability, including: the use of lasers having a high degree of wavelength stability; the use of lasers that generate infrared radiation so as to reduce fluorescence background problems; the use of radiation filtering devices to adequately reject the elastically scattered photons; the use of multi-dimensional charge coupled devices (CCD) for discerning extremely low levels of radiation; the use of a beam splitter to simultaneously irradiate a sample and a reference material in order to compensate for variabilities in the apparatus; the use of integral transform techniques for improved signal processing by removing undesirable frequency wanderings and intensities from the spectral data; the use of simultaneous data measurement of the excitation source and the Raman beam for precise arithmetic calculations; and, the use of beam monitoring at various points in the optical path for enabling higher precision and accuracy in the arithmetic calculations.
While existing apparatus for producing Raman spectrum may be suitable for their intended purpose, there still remains a need in the art for an apparatus and method for producing a calibrated Raman spectrum having a high degree of accuracy and repeatability.